1. Field of the Invention
This invention relates to improvements in a method of producing a washer having a boss, from a bimetal material which is a composite material including different metals, and more particularly to a heat treatment in such a method.
2. Description of the Prior Art
Hitherto it is known that a washer having a boss is formed of a composite material such as a bimetal material including a bearing alloy and an iron-based alloy. Such a bimetal material is conventionally produced by bonding different metals by hot cladding, cold cladding, sintering or the like. The bimetal material as a material for a washer having a boss is usually produced by sintering. Additionally, a heat treatment is conducted in the process for producing the bimetal material by the sintering and in a process for producing the washer having the boss from the bimetal material. Such heat treatment is conventionally conducted as follows:
First, powder of the bearing alloy is sprinkled on a backing metal and rolled to form a composite material. The composite material is subjected to a primary sintering (annealing) and then to a primary rolling. The composite material is thereafter subjected to a secondary sintering to form a bimetal material. A secondary rolling is conducted on the bimetal material so as to flatten the bimetal material. A blank of the shape of a washer is punched out from the bimetal material by using a press machine. The thus formed blank is formed into a washer having a boss through a burring step. Thereafter, the backing metal is carburized in a carburizing step thereby to strengthen the backing metal.
However, drawbacks have been encountered in the above discussed conventional production method of the washer having the boss, in which the washer having the boss is fabricated through the primary sintering and the secondary sintering, and additionally carburizing for the backing metal is thereafter carried out. Accordingly, the conventional production method of the washer having the boss has been unavoidably includes much steps and therefore is degraded in productivity and difficult in quality control, raising production cost of the washer having the boss.